This disclosure relates to polycarbonates comprising aliphatic diols, and in particular to isosorbide-based polycarbonates, and methods of manufacture thereof.
Polymers based on aliphatic diols derived from biologically-based sources are of great interest in the plastics industry and for the manufacturing industry, for the preparation of materials and products that can be derived from inexpensive, renewable sources and that also are biodegradable, and thereby have a low net environmental impact. Of particular interest are polymers based on isosorbides, and more specifically referred to as 2,6-dioxabicyclo[3.3.0]octan-4,8-diol, 1,4:3,6-dianhydro-D-glucitol, and 2,3,3a,5,6,6a-hexahydrofuro[3,2-b]furan-3,6-diol, and isomers of these. These materials are of great interest to the chemical industry, and in particular in the production of polymeric materials such as polycarbonates, because such aliphatic diols can be produced from renewable resources, namely sugars, rather than from the petroleum feed stocks used to prepare other monomers useful in the production of polycarbonates, such as bisphenol monomers.
However, for practical applications, polycarbonate incorporating isosorbide needs a balance of properties to be useful. Polycarbonates in general must have sufficiently high molecular weight for desirable mechanical properties, and sufficiently low glass transition temperatures and flow to be useful in molding and extrusion applications. A problem that accompanies inclusion of such biologically derived materials in polycarbonates is maintaining the desired mechanical and optical properties of the polycarbonate during and after high temperature processing, such as encountered during extrusion and molding, where. Polycarbonate that include isosorbide that otherwise have desirable properties of molecular weight can, under extrusion or molding conditions, exhibit undesirable degradation and commensurate increases in undesired color change and decreases in molecular weight. While the former has an undesirable effect on appearance, the latter can adversely affect the melt flow and mechanical properties of the polycarbonate.
A typical solution to the desired balance of molecular weight and melt flow has been to include “soft blocks”, which are segments of lower glass transition temperature compositions, such as for example segments of polycarbonate based on resorcinol, interspersed in the polycarbonate. Doing so can reduce the net glass transition temperature of the polycarbonate, and increase the melt flow. Typical soft blocks include resorcinol, aliphatic diols and aliphatic diacids. Of these, it is desirable to include an aliphatic soft block because of the transparency of such soft blocks to ultraviolet light (UV), and hence inherent UV stability of aliphatic blocks; however aliphatic soft blocks are typically difficult to incorporate into polycarbonates thereby making it difficult to obtain polymers of sufficiently high molecular weight. In addition, difficulty in incorporating these soft blocks translates to differences in reactivity of the soft block and other monomers, which can lead to the formation of block copolymers instead of random copolymers. In turn, block copolymer formation can lead to phase separation of the dissimilar blocks, which negates the desired overall Tg reduction by creating regions having distinct glass transition temperatures, which can in turn have adverse effects on the soft block-containing copolycarbonate such as phase separation leading to haze and processing problems such as delamination.
There accordingly remains a need in the art for an isosorbide-based polycarbonate having a sufficiently high molecular weight and heat resistance while having a sufficiently low glass transition temperature to allow polymerization and processing and at the same time preventing phase separation and degraded mechanical properties in the composition.